In order to better utilize trucking equipment, truck bodies which can be disconnected and later reconnected to the truck were developed. By this means, the truck as a whole and the driver need not be tied up waiting for the truck to be filled or emptied. Such methods have been used for many years with tractor-trailer types of trucks since the trailer may simply be separated from the tractor and reattached as needed. One disadvantage of the tractor-trailer approach is that each trailer has to be equipped with axles, tires, brakes, and lights. However, the expense of these accessories is justified by the large loads that can be carried. Where smaller loads are involved, it has not been found economical to either provide smaller trailers or to carry smaller loads in or on full sized trailers. Thus, trucks with off-loadable containers have been provided for such smaller loads.
One system for handling such smaller loads which has found wide acceptance is the so-called roll-off arrangement. In a roll-off container arrangement, a solid (or non-trailer) truck chassis has a hydraulically tiltable frame and usually a cable system for drawing the container onto the tilt frame and off-loading the container. Either the tilt frame, the container, or both have rollers and guide tracks to facilitate movement of the container on the frame. The tilt frame is tilted up and back for container loading and is positioned horizontally for transportation of the container. Originally, drum winches were employed for movement of the cable connected to the container. However, it was found that friction and pressure between portions of the cable on the drum was destructive to the cable which, therefore, had to be replaced too frequently. To overcome this problem, lifting arrangements employing linear hydraulic motors or rams and pulleys were developed.
Linear motor roll-off arrangements usually employ multiple pulleys to multiply the travel rate of the cable for a given ram stroke length. This allows shorter rams which economizes the mechanism in a number of ways, including: lower initial cost for the ram or rams and the hydraulic fluid reservoir since less fluid is required to fully extend the shorter cylinders. In order to conserve space on these mechanisms when multiple pulleys are employed, it has been the usual practice to orient all but the final pulley for rotation in approximately the same plane as the tilt frame. That is, when the frame is tilted to horizontal, the pulleys are rotating in a horizontal plane. The final pulley before the cable connects to the container, is usually oriented for rotation in a vertical plane.
One problem with the horizontally oriented pulleys is that when the cable is in a slack condition and not tensioned about the pulleys, the cable tends to slip off the pulleys under the influence of gravity and, possibly, jolts and vibrations. If the cable slips off the pulleys, it might become caught or wedged in other portions of the mechanism. The only solution then is to manually re-reeve the cable properly about the pulleys. Even if the cable is not caught in this manner, there is the possibility of frictional damage to the cable by scraping against portions of the mechanism as it is tensioned.
The transition of the cable from a horizontally oriented pulley to the final vertically oriented pulley places side loads on these pulleys which can result in binding and accelerated wear of their bearings.